Electromagnetic machines e.g. motors, generators or the like, require two independent types of overcurrent protection including the one against an excess electromagnetic force (a mechanical force) generated by an excess electric current flowing therein even during a short period and the other against an overheating caused by accumulation of heat generated by electric current flowing therein during a relatively long period even if the intensity of the electric current is relatively small.
A traditional means for the former is a moving iron type overcurrent relay which quickly reacts to an overcurrent, and a traditional means for the latter is an induction type overcurrent relay which has characteristics to integrate the results of the electric current flowing therein.
Development of automatic control systems applicable to electric machines e.g. development of a pulse width modulation system applicable to electric machines, has caused a tendency to design such overcurrent protection means by means of an analog or digital circuit system which is designed in one body with the aforementioned automatic control system of an electric machine, rather than by means of the aforementioned electromagnetic equipment.
Referring to FIG. 1, one example of such overcurrent protection means which employs an analog or digital circuit system will be described below. An electric current sensed by an electric current sensing means 6 is applied to an excess electric current sensing means 7 realized by employing a bistable multivibrator or the like e.g. a Schumit circuit or the like. An integration circuit 8 consisting of for example, an RC circuit exponentially accumulates the electric charge continuously applied by the excess electric current sensing means 7, in the capacitor thereof, and the quantity of electric charge stored therein exponentially increases during the period wherein the excess electric current sensing means 7 continues to output a signal. An alarm circuit 9 which is also a bistable multivibrator or the like e.g. a Schumit circuit or the like, continues to output an alarm signal during the period in which the output voltage of the integration circuit 8 exceeds a predetermined value.
Such an overcurrent detector as described above or an overcurrent detector consisting of analog or digital circuits for example, is remarkably advantageous, in the cases where the control circuits of electric machines to be protected also contain analog or digital circuits which are similar to those which are employed for the aforementioned overcurrent detector employing analog or digital circuits. This is because considerable parts of the automatic control circuits are allowed to be employed also for the circuits of the overcurrent detection.
However, if the control circuit of an electric machine is designed employing a computer, the aforementioned advantage can not be as large as was described above. This is because the aforementioned overcurrent detector alone is required to be designed employing plural discrete electronic elements for producing analog or digital circuits.
Therefore, the aforementioned tendency to produce the control circuit of electric machines by employing a computer has derived a desire to develope an overcurrent detector employing a computer.